The present invention relates generally to the field of metal plating. In particular, the present invention relates to the field of plating metal bumps, such as solder bumps, on electronic devices.
Thick photoresist layers, e.g. from 5-10 μm up to 50-100 μm, are used in certain electronic device manufacturing processes. For example, thick photoresist layers are used in high volume applications for micro electro mechanical systems (“MEMS”), in the fabrication of molds for metal deposition in chip scale packaging, solder paste printing, TAB wire bonding and the like. In particular, thick photoresist layers are useful in the formation of large mechanical structures such as solder bumps on semiconductor wafers.
In such processes, a substrate is coated multiple times with sufficient photoresist to provide a thick photoresist layer. Such thick photoresist layers are then imaged according to conventional procedures to provide the desired features or patterns. The imaged layers are then developed using the appropriate procedures for the particular photoresist employed. Following development, metal such as copper, tin, lead, gold or a mixture of any of these is deposited into the features. After metal deposition, the thick photoresist is removed and the metal deposit may optionally be reflowed to provide a somewhat spherical shape for chip scale packaging.
In such conventional processes, thick photoresist layers are achieved by using multiple layers of conventional photoresists. Conventional photoresists provide a photoresist layer having a thickness of about 1-10 μm. Many such coating steps are required to build up a photoresist layer of 15-100 μm. Each step adds to the time and monetary cost of the process and the resulting electronic devices. There is accordingly a need for a method of applying a thick photoresist layer in a single step.
U.S. Pat. No. 4,592,816 (Emmons et al.) discloses compositions and a process for the electrophoretic deposition of a photoresist. The photoresists of this patent are aqueous based and contain a (meth)acrylate binder polymer having positively or negatively charged groups, and a cross-linking agent. Very thick photoresist layers, i.e. ≧50 μm, are not disclosed in this patent.
Certain non-aqueous liquid photoresists are commercially available that contain a (meth)acrylate binder polymer having non-charged amino groups, an acrylate cross-linking agent, and a photoactive component. Such photoresists have a total solids content of less than 35%. Thick photoresist layers cannot be made in a single coating procedure using these commercially available photoresists.
Korean Patent Application No. 10-1999-0039084, to Samsung Electronics Co., discloses a method for applying a photoresist to a semiconductor wafer. The photoresist layer has a thickness of up to about 35,000 Å (or 3.5 μm). This is achieved by spraying the photoresist on the wafer, rotating the wafer at a relatively low speed and a low acceleration rate to harden the photoresist, and then rotating the wafer at a high rotation speed and a high acceleration rate to provide the photoresist layer having the desired thickness and uniformity. Very thick photoresists are not disclosed in this patent application.